In wireless communication, the effect of multipath fading can be lessened by employing diversity techniques. One such technique is antenna diversity where multiple antennas are used at the mobile terminal to receive several independently fading signals from the base station. Another technique, called transmit diversity, involves time-shifting the signal at the base station, then transmitting it to the mobile terminal on multiple antennas. The time-shifted versions of the signal create frequency selective fading that may then be equalized at the receiver to provide the diversity gain.
Transmit diversity is used in most modern wireless communication systems, including 3GPP (3rd Generation Partnership Project), which is a WCDMA (wideband code division multiple access) based system. In WCDMA base stations, four modes of operations are specified: (1) no transmit diversity, (2) open loop transmit diversity, (3) closed loop transmit diversity mode 1, and (4) closed loop transmit diversity mode 2. The first mode of operation, of course, involves no transmit diversity. The second mode of operation is typically used when the mobile terminal is moving very rapidly. The third and fourth modes of operation are typically used when the mobile terminal is moving more slowly. These modes of operation are generally known in the art and will not be described in detail here (see, e.g., 3GPP Specification TS 25.214 V5.8.0 (2004-03)).
In the closed loop transmit diversity modes, the channel condition is transmitted from the mobile terminal on the uplink to the base station in the form of feedback information (FBI). The FBI may then be used to adjust the phase and amplitude (i.e., beam-forming) of the dedicated physical channel (DPCH) transmitted to the mobile terminal on the downlink. By using beam-forming, improved performance in terms of lower downlink code power for the same Quality of Service (QoS) may be achieved. This is especially true for low Doppler frequencies when the vehicular velocity of the mobile terminal is low. As a result, the downlink capacity may also be improved as compared to the open loop transmit diversity mode of operation.
Complications may arise, however, when closed loop transmit diversity is used in soft handover situations, primarily because the mobile terminal can receive signals from more than one base station in such soft handovers. WCDMA systems rely on strict power control to maintain signal quality such that the link between the mobile terminal and the base station typically uses only enough power to achieve the required QoS and no more. Moreover, WCDMA power control is designed such that only one uplink will meet the required QoS at any given moment. Thus, the quality of the link between the mobile terminal and the other base stations involved in the soft handover will be much worse. Consequently, detection of the FBI on the uplink in these other base stations may be erroneous, resulting in erroneous antenna adjustments on the downlink.